The present invention relates to an image forming apparatus of the type causing a developer deposited on a developer carrier to rise in the form of a magnet brush in a developing region and develop a latent image formed on an image carrier.
It is a common practice with a copier, printer, facsimile apparatus or similar electrophotographic or electrostatic image forming apparatus to electrostatically form a latent image on an image carrier in accordance with image data. The image carrier may be implemented by a photoconductive element or a photoconductive belt. A developing device develops the latent image with toner and thereby produces a corresponding toner image. A current trend in the imaging art is toward a magnet brush type developing system using a toner and carrier mixture or two-ingredient type developer. This type of developing system is desirable from the standpoint of image transfer, halftone reproducibility, and stability of development against varying temperature and humidity. Specifically, a developing device using this type of system causes the developer to rise in the form of a brush chain on a developer carrier, so that toner contained in the developer is transferred to a latent image formed on the image carrier at a developing region. The developing region refers to a range over which a magnet brush rises on a developer carrier and contacts the image carrier.
The developer carrier is generally made up of a hollow cylindrical sleeve or developing sleeve and a magnet roller surrounded by the sleeve. The magnet roller forms a magnetic field for causing the developer deposited on the sleeve to rise in the form of a head. When the developer rises on the sleeve, carrier particles contained therein rise along magnetic lines of force generated by the magnet roller. Charged toner particles are deposited on each of such carrier particles. The magnet roller has a plurality of magnetic poles formed by rod-like magnets and including a main magnetic pole for causing the developer to rise in the developing region.
In the above configuration, when at least one of the sleeve and magnet roller moves, it conveys the developer forming a head thereon. The developer brought to the developing region rises in the form of a brush chain along the magnetic lines of force generated by the main magnetic pole. The brush chain or head contacts the surface of the image carrier while yielding itself. While the brush chain or head sequentially rubs itself against a latent image formed on the image carrier on the basis of a difference in linear velocity between the developer carrier and the sleeve, the toner is transferred from the developer carrier to the image carrier.
It has been customary to apply a lubricant to the image carrier or a process unit around it for insuring high quality images over a long time. If the image carrier has a great coefficient of friction, then vermicular omission occurs in an image portion where much toner is deposited, e.g., at the center of a line image at an image transfer stage. The ratio of such local omission noticeably varies in accordance with the fluidity of the toner that is dependent on, e.g., environment. Further, at a cleaning stage, a cleaning blade is entrained by the image carrier and fails to clean the image carrier. This not only cause black stripes to appear in an image, but also causes the cleaning blade to wear at an unexpected rate. By applying a lubricant to, e.g., the image carrier, it is possible to reduce friction acting between the image carrier and the cleaning blade and between the image carrier and an image transferring member and therefore to reduce the peel-off of the photoconductive layer of the image carrier. The lubricant therefore solves the above problems and extends the life of the image carrier. In addition, the lubricant obviates annoying sound.
However, the problem with the lubricant is that it lowers the coefficient of friction of the image carrier and therefore the amount of toner to deposit on the image carrier, preventing sufficient image density from being achieved. To solve this problem, tonality must be corrected by varying a bias for development or the power of a laser beam. Such correction needs extremely sophisticated control and therefore increases cost. Further, when adhesion between the toner and the image carrier and the force of the magnet brush rubbing the image carrier are brought out of balance, dots forming a halftone portion are locally lost, resulting in a granular image. Moreover, a ratio of the linear velocity of the sleeve to that of the image carrier cannot be increased because the trailing edge of a halftone image would be lost due to counter charge and the force of the magnet brush acting on the carrier.
Japanese patent application Nos. 11-39198, 11-128654 and 11-155378, for example, propose image forming apparatuses constructed to protect even a low contrast image from the omission of a trailing edge for thereby insuring desirable image density and quality. However, there is an increasing demand for an image forming apparatus capable of further improving image density and quality.
Technologies relating to the present invention are also disclosed in, e.g., Japanese patent laid-open publication Nos. 5-257387, 8-101584, 8-202226, 9-34261, 9-127793, 2000-10419, 2000-19858, 2000-47523, 2000-47524, and 2000-305360.
It is therefore an object of the present invention to provide an image forming apparatus capable of insuring a uniform halftone image, preventing the trailing edge of an image from being lost, and faithfully reproducing even a horizontal line.
In accordance with the present invention, an image forming apparatus includes a developing device including a main magnetic pole for causing a developer to magnetically deposit on the outer periphery of a developer carrier in the form of a magnet brush. An image carrier is located to face the developing device. The image carrier has a coefficient of friction of 0.5 or below. A flux density in the normal direction has an attenuation ratio of 40% or above, as measured in a developing region where the magnet brush contacts the image carrier.